Rollup merge of #82464 - ehuss:unix-command-comment, r=kennytm

Update outdated comment in unix Command.

The big comment in the `Command` struct has been incorrect for some time (at least since #46789 which removed `envp`). Rather than try to remove the allocations, this PR just updates the comment to reflect reality. There is an explanation for the reasoning at #31409 (comment), discussing the potential of being able to call `Command::exec` after `libc::fork`.  That can still be done in the future, but I think for now it would be good to just correct the comment.

library/std/src/sys/unix/ext/process.rs

@@ -172,6 +172,8 @@ impl CommandExt for process::Command {
     }
 
     fn exec(&mut self) -> io::Error {
+        // NOTE: This may *not* be safe to call after `libc::fork`, because it
+        // may allocate. That may be worth fixing at some point in the future.
         self.as_inner_mut().exec(sys::process::Stdio::Inherit)
     }
 

library/std/src/sys/unix/process/process_common.rs

@@ -60,25 +60,13 @@ cfg_if::cfg_if! {
 ////////////////////////////////////////////////////////////////////////////////
 
 pub struct Command {
-    // Currently we try hard to ensure that the call to `.exec()` doesn't
-    // actually allocate any memory. While many platforms try to ensure that
-    // memory allocation works after a fork in a multithreaded process, it's
-    // been observed to be buggy and somewhat unreliable, so we do our best to
-    // just not do it at all!
-    //
-    // Along those lines, the `argv` and `envp` raw pointers here are exactly
-    // what's gonna get passed to `execvp`. The `argv` array starts with the
-    // `program` and ends with a NULL, and the `envp` pointer, if present, is
-    // also null-terminated.
-    //
-    // Right now we don't support removing arguments, so there's no much fancy
-    // support there, but we support adding and removing environment variables,
-    // so a side table is used to track where in the `envp` array each key is
-    // located. Whenever we add a key we update it in place if it's already
-    // present, and whenever we remove a key we update the locations of all
-    // other keys.
     program: CString,
     args: Vec<CString>,
+    /// Exactly what will be passed to `execvp`.
+    ///
+    /// First element is a pointer to `program`, followed by pointers to
+    /// `args`, followed by a `null`. Be careful when modifying `program` or
+    /// `args` to properly update this as well.
     argv: Argv,
     env: CommandEnv,